Electrophotography is a useful process for printing images on a receiver (or “imaging substrate”), such as a piece or sheet of paper or another planar medium, glass, fabric, metal, or other objects as will be described below. In this process, an electrostatic latent image is formed on a photoreceptor by uniformly charging the photoreceptor and then discharging selected areas of the uniform charge to yield an electrostatic charge pattern corresponding to the desired image (a “latent image”).
After the latent image is formed, toner particles are given a charge substantially opposite to the charge of the latent image, and brought into the vicinity of the photoreceptor so as to be attracted to the latent image to develop the latent image into a visible image. Note that the visible image may not be visible to the naked eye depending on the composition of the toner particles (e.g. clear toner).
After the latent image is developed into a visible image on the photoreceptor, a suitable receiver is brought into juxtaposition with the visible image. A suitable electric field is applied to transfer the toner particles of the visible image to the receiver to form the desired print image on the receiver. The imaging process is typically repeated many times with reusable photoreceptors.
The receiver is then removed from its operative association with the photoreceptor and subjected to heat or pressure to permanently fix (“fuse”) the print image to the receiver. Plural print images, e.g. of separations of different colors, are overlaid on one receiver before fusing to form a multi-color print image on the receiver.
Electrophotographic (EP) printers typically transport the receiver through a fuser which provides heat to fix the print image to the receiver. However, if the receiver transport is interrupted and the receiver is left stationary close to a heat source, the receiver can be damaged or ignite, possibly causing damage and injury. Various schemes have been proposed to mitigate these dangers.
Moore, in U.S. Pat. No. 3,922,520, describes a radiant fuser including heating elements surrounded by a high-heat-capacity material in a housing. To fuse, the housing opens, and the stored heat from the high-heat-capacity material, and energy provided by the heating elements directly to the receiver, heat the receiver to melt the toner of the print image thereon. However, this scheme requires mechanically closing the housing when the receiver is stationary near the fuser, which increases the risk of damage to the filaments of lamps used as heating elements.
Billet et al., in U.S. Pat. No. 5,526,108, describe a radiant fuser having radiant sources mounted in hinged housings. The housings close to shield a stationary receiver from energy produced by the radiant sources. However, this scheme requires complex mechanical systems to move the panels. Furthermore, receiver stoppages are unexpected events, so the motion must be swift to reduce danger. Sudden motions can damage the radiant energy sources.
Both of these techniques require mechanical motion to shield the receiver from the energy source. The tracks on which mechanical parts ride can wear or gum, bearings can freeze, springs can break, and in other ways these mechanical systems can become unable to perform their safety functions. Furthermore, in all of these schemes, loss of electrical power to control the safety functions can result in the failure of those safety functions to protect the receiver.
There is a continuing need, therefore, for an improved toner heating device which does not require mechanical motion or electrical power to safeguard against damage and injury due to overheating of receivers.